Dry. Saturated. 1 litre . . . 1.108 grm. . . 1.102 grm. 1 gramme . . . 0.902 litre. . . 0.907 litre. 1000 cubic feet . 69.18 lb. . . . 68.83 lb. It should be remembered that unless the gas has been passed through a chemical drier, it is always saturated with aqueous vapour, the amount of water present being governed by the temperature and pressure. The 1 litre of moist acetylene which weighs 1.102 gramme at 60° F. and 30 inches of mercury, contains 0.013 gramme of water vapour; and therefore the weight of dry acetylene in the 1 litre of moist gas is 1.089 gramme. Similarly, the 68.83 pounds which constitute the weight of 1000 cubic feet of moist acetylene, as measured under British standard conditions, are composed of almost exactly 68 pounds of dry acetylene and 0.83 pound of water vapour. The data required in calculating the mass of vapour in a known volume of a saturated gas at any observed temperature and pressure, _i.e._, in reducing the figures to those which represent the dry gas at any other (standard) temperature and pressure, will be found in the text-books of physical chemistry. It is necessary to recollect that since coal-gas is measured wet, the factors given in the table quoted in Chapter XIV. from the "Notification of the Gas Referees" simply serve to convert the volume of a wet gas observed under stated conditions to the equivalent volume of the same wet gas at the standard conditions mentioned. HEAT OF COMBUSTION, &C--Based on Berthelot and Matignon's value for the heat of combustion which is given on a subsequent page, viz., 315.7 large calories per molecular weight of 26.016 grammes, the calorific power of acetylene under different conditions is shown in the following table: Dry. Dry. Saturated. 0° C. & 760 mm. 60° F & 30 ins. 60° F. & 30 ins. 1 gramme 12.14 cals. 12.14 cals. 12.0 cals. 1 litre 14.l9 " 13.45 " 13.22 " 1 cubic foot 40.19 " 380.8 " 374.4 "